1. Field of the Invention
The present invention relates to an image forming apparatus using an electro-photography or electrostatic imaging method, especially to an image forming apparatus for a copier, printer or facsimile.
2. Related Background Art
Conventionally, in an image forming apparatus for a copier, laser printer or facsimile, such a fixing apparatus is known to be constructed of a fixing roller equipped therein with a heating source such as halogen lamp and a pressure roller equipped likewise with a heat source and pressure-mounted onto the fixing roller, feed through a recording sheet holding an unfixed toner image thereon between the fixing and the pressure rollers, let the softened toner by the above described fixing roller seep into the gaps among fibers of the recording sheet and fix the unfixed toner image onto the recording sheet.
The halogen lamps as heat source for the fixing and the pressure rollers are on- and off-controlled respectively by the individual signals from a temperature sensor each mounted on the surface of the fixing and the pressure rollers, and thereby the each surface is controlled at a preset temperature.
In such constructed fixing apparatus utilizing a heat roller method, a pressure point is formed between the two rollers (hereinafter referred to as “nip”) due to an elastic deformation of the elastic layer of the fixing roller. And in such a fixing apparatus, the unfixed toner image is melted and fixed onto the recording sheet by the thermal energy and the pressure by feeding a recording sheet carrying an unfixed image thereon through the nip.
In such constructed fixing apparatus as described above, the fixing and the pressure rollers has an elastic layer which consists of elastic body such as silicone rubber having a certain thickness and therefore its coefficient of thermal conductivity being relatively low. Consequently, in the above described fixing apparatus, if the surface temperature on the fixing or the pressure rollers decreases when the recording sheet is fed, a time lag may occur between the time when the halogen lamp is turned on by the temperature detection signal from the sensor and the time when the halogen lamp heat is conducted through the elastic layer to the surface, possibly causing a fixing failure. Especially with a higher process speed, such a fixing failure is prone to occur and a problem may result such as a limited continuous printing speed in order to avoid such a fixing failure.
As a solution for such problem, a technique has already been proposed to alleviate a temperature drop on a fixing roller surface by pressure-mounting an external heat roller with its temperature being maintained high, as disclosed in Japanese Patent Application Laid-Open Nos. H10-149044 and H11-24489.
However, such fixing apparatus has had a problem of damage or degradation in the fixing roller if the external heating roller keeps contact with a stationary fixing roller, due to a continuous, local heating of the fixing roller by the external heating roller even if the surface temperatures of the both rollers are controlled within each applicable temperature. Such degradations in the fixing roller surface cause problems including image defects such as image streaks and gloss defects, toner adhesion (offset) due to a decreased toner releasing capability and recording paper stripping failures.
As a solution for such problems, a technique has already been proposed in which an external heating roller is set up detachably with a fixing roller and the fixing roller is rotated load-free by rotation drive means until the external heating roller comes in contact therewith, as disclosed in Japanese Patent Application Laid-Open No. H11-24461.
Also in the conventional image forming apparatus described above, an interlock function is equipped as protection means for stopping the drive system by shutting the electric power off the load system in such cases where the machine operator opens (hereafter referred to as “door opening”) an external cover (hereafter referred to as “front door”) for a jam removal and the like.
Such image forming apparatus, however, has problems as follows when the interlock function is in operation by a door opening.
If the interlock function is in operation by a door opening with the external heating roller in contact with the fixing roller, both of the rollers are forced to stop rotating instantly, and the drive means detaching the external heating roller from the fixing roller is also forced to be inoperable, causing a part of the fixing roller contacting with the external heating roller to be locally heated thereby, if the heater mounted in the external heating roller is kept powered on, resulting in the local degradation of the fixing roller surface.
Particularly because the temperature of the external heating roller is maintained higher than that of the fixing roller, such local damage is unavoidable, hence requiring a definite solution.